3.1 In General
Dense wavelength-division multiplexing (DWDM) is currently the preferred method for satisfying bandwidth demand for fiber-optic long-haul transport in telecommunication systems. As the optical data signal travels through the fiber, however, fiber nonlinearities, fiber loss, and amplified spontaneous emission (ASE) all act to degrade the signal-to-noise ratio of the data signal. As a result, the optical signal needs to be periodically amplified and eventually electrically regenerated so as to maintain the signal integrity.
The design of an economical DWDM network has to consider performance versus cost for all of the network elements such as transmitters, amplifiers, regenerators, add/drop nodes, receivers, etc. To save expense on the amplifiers, for example, the length of the amplifier span should be as long as possible. Extended amplifier span, however, results in high ASE accumulation from low input to amplifiers, and/or large nonlinear effects due to the use of high launch power. This degradation in turn demands more frequent optical-to-electrical-to-optical (OEO) regeneration. A typical 40 channel DWDM system using Erbium-doped fiber amplifiers (EDFA's) generally has an amplifier span spacing of about 80 km and OEO regeneration distances of around 500–600 km.
Currently, OEO regeneration is responsible for a much larger percentage of overall system cost than amplifiers. It is, therefore, desirable to extend the regeneration distance wherever possible.
Raman fiber amplifier technologies have emerged as a highly promising building block to allow DWDM transmission for distances greater than 1500 km without OEO regeneration. This is particularly important inasmuch as the majority of data traffic on a telecommunications network typically has a destination distance which exceeds 1000 km.
Raman fiber amplifiers generally use the transmission fiber itself as the gain medium. Thus, the signal passing through the fiber is amplified as it propagates. Because of their distributed nature, Raman amplifiers present several significant advantages over EDFA technologies:
(1) Since the Raman amplification makes the fiber partially transparent, a high signal-to-noise ratio can be maintained over multiple span distances.
(2) For the same reason, the nonlinear effects can be significantly reduced since the maximum signal power in the fiber can be significantly reduced.
(3) The Raman gain profile of optical fiber is much broader and smoother than that of EDFA's. As a result, the combination of several pump laser wavelengths can provide overall gain profiles as broad as near 100 nm with less than 1 dB gain ripple, thus reducing the gain ripple and tilting effects generally associated with EDFA's.
(4) Inline EDFA's typically operate in saturation mode where the gains of the amplifier for individual wavelength channels are a function of total input signal power. As a result, when an optical channel is added or dropped from the fiber, the gains for all of the other channels change, causing network transients, distortions and misbalances. By contrast, Raman amplifiers typically work in a linear mode where the gain of each channel is independent of all of the other channels. Thus, dynamic wavelength adds/drops in a Raman system have less of an effect on overall system performance.
Raman-assisted transmission generally compares favorably to EDFA-only systems, leading to better system quality factors, longer amplifier spans, and hence longer DWDM transmission before the need for OEO regeneration.
One evolving application area of Raman amplifiers, beyond the traditional long-haul transmission application discussed above, is in the hybrid fiber coaxial cable (community antenna) TV (HFC-CATV) industry. HFC-CATV, using subcarrier multiplexing (SCM) and multilevel quadrature amplitude modulation (QAM) technology, generally requires >50 dB carrier-to-noise ratio and high suppression of inter-modulation, second order and third order distortions, which generally calls for high optical power and low noise accumulation. Currently, this is typically achieved by employing high output power EDFA's (up to 25 dBm) and limiting span spacing (typically 30–40 km) and amplifier stages. However, by using distributed Raman amplification, it is possible, due to the low noise figure (NF) and in-fiber Raman amplification, to extend the span spacing, add amplifier stages and lower input power without significantly compromising system performance. However, applications of Raman amplifiers in the CATV industry are currently impaired by cost.
Improvements in Raman amplification would also have advantageous applications in a wide range of instrumentation and imaging applications.
3.2 Principles of Raman Amplification
The principles of Raman amplification in fiber are based on the process of stimulated Raman scattering (SRS). When an optical beam of frequency ωP is injected into, and propagates along, an optical fiber, a small fraction (typically 10−6) of the incident photons are scattered by the molecules to lower-frequency (ωS<ωP) states while, at the same time, the molecule makes a transition between vibrational states. The incident light acts as a pump to generate the frequency down-shifted (ωS) light, which is sometimes called Stokes waves. If a signal at the frequency ωS is coincident with the pump beam (at frequency ωP) in the fiber, the signal will be amplified by the Raman scattering-induced Stokes waves (also at frequency ωS).
One outstanding feature of fiber Raman amplifiers is that the Raman gain profile of standard fused silica fiber is very broad due to the amorphous nature of the glass. Typically, the Raman gain increases nearly linearly from the pump frequency towards a maximum at about 13.2 THz (100 nm) and then falls off fairly sharply. For example, if a high power 1450 nm laser is injected into a 40 km standard single mode fiber such as Corning SMF-28™ fiber, the Raman gain profile will have a maximum near 1550 nm, with a usable gain bandwidth of ˜30 nm (see FIG. 1).
The magnitude of the Raman gain in optical fiber depends on the respective compositional doping elements. The normalized gain spectral shape, however, is much less sensitive to glass composition for most fibers typically used.
In the first order approximation, the Raman gain increases nearly linearly, in dB, with increasing pump power. Nearly 10 dB of Raman gain is typical for 500 mW of pump power injected into 40 km of Corning SMF-28™ fiber. The upper limit of realistically useful Raman gain is generally limited by the double Rayleigh scattering process. At that upper limit, the Rayleigh back scattering causes multiple reflections of both the ASE and the signal, thus causing performance degradation of the transmission. Double Rayleigh scattering becomes prominent as the Raman gain reaches above 15 dB.
Due to the fact that the Raman gain profile is determined by the pump laser wavelength, the Raman gain spectrum can be tailored in shape and width by selecting the appropriate pump laser wavelength spectrum. For telecommunication applications, it is common to combine a number of high power diode lasers, each with a different wavelength, to collectively yield a broad and flat gain profile so as to accommodate the transmission window of interest. To construct a 100 nm bandwidth Raman amplifier, up to 8 pump wavelengths, ranging from 1430 nm to 1520 nm, are needed for the amplification of signals in the 1530 nm to 1620 nm band with less than 1 dB gain ripple. The 1 dB gain ripple is due in part to the fine and sharp structures near the peak of the gain profile shown in FIG. 1.
One significant characteristic of Raman amplifiers is that Raman amplification is effective only if the signal beam has the same polarization as that of the pump. To obtain a polarization-insensitive Raman amplifier, two diode lasers, with orthogonal polarization, are generally used for each pump wavelength. Alternatively, another approach to solve this issue is to use various polarization scrambling techniques. However, current polarization scrambling techniques are generally relatively costly and bulky, and have unproven reliability.
Another important issue for distributed Raman amplification is the relative intensity noise (RIN) transfer from the pump laser to the signal. Raman scattering, due to its fast response time, causes amplitude noise in the pump lasers to be proportionally transferred to the gain fluctuations. As is schematically shown in FIG. 2, when a noise-free signal beam propagates along the Raman amplified transmission fiber, the signal data set experiences a time-dependent amplification. As a result of this phenomena, the output signal beam carries amplitude noise.
The noise transfer from pump laser to the signal beam depends on the pump geometry employed.
More particularly, if the pump propagates in the direction opposite to that of the signal, the signal beam experiences the gain through its entire traveling time in the fiber, and the RIN of the pump laser is then effectively averaged over the travel time. For a fiber length of 20 km, the signal beam traveling time is about 100 microseconds. The high frequency RIN components, typically above 1 MHz, are substantially insensitive to the signal transmission quality (FIG. 3). This suppression of the RIN transfer from pump to signal is one of the main reasons that “backward pumping” remains the most popular scheme for Raman amplification in telecommunication systems.
On the other hand, when a forward pump scheme is employed so as to provide a co-propagating pump and signal, the average of the pump RIN is determined only by the walk-off time between the signal and pump generated by dispersion of the fiber. The RIN transfer from pump to signal remains effective for much higher frequency components, as shown in FIG. 3. The RIN of the pump lasers must then generally be limited to <−150 dB/Hz. This requirement is often difficult to achieve for the commonly-used frequency-stabilized Fabry-Perot lasers. To achieve such low RIN, distributed feedback (DFB) lasers have generally been used for forward pumping applications. A disadvantage of using DFB lasers as a Raman pump, however, is that the narrow linewidth of a DFB laser causes significant Brillouin back-scattering at a power level much lower than that needed for the Raman pump. As a result, linewidth broadening techniques need to be applied to suppress the Brillouin scattering while keeping the RIN within the required range.
When multiple high power diode lasers are used as the Raman pumping source, nonlinear interactions between the various pumps becomes an important design consideration. When the frequencies of the pump lasers vary over 100 nm, the pumps at high frequency can effectively amplify the pumps at lower frequencies (see FIG. 4), causing nonlinear power evolution of the pumps inside the fiber. In addition, four-wave-mixing between pumps can create side bands with frequencies extending into the signal band. This causes an excessive noise floor for certain signal channels.
Generally, distributed Raman amplifiers are currently associated with high cost because of their sophisticated design, expensive high power diode lasers and complex packaging. More fundamentally, technical limitations such as pump-pump nonlinear interactions and gain ripple are inherently associated with the use of multiple high power diode lasers to provide a relatively broad and flat gain profile.